Fix clang 3.5.0 warnings in usr.sbin/rtadvd

[dim]

Summary

Fix the following -Werror warnings from clang 3.5.0, while building
usr.sbin/rtadvd:

usr.sbin/rtadvd/rtadvd.c:1291:7: error: taking the absolute value of unsigned type 'unsigned int' has no effect [-Werror,-Wabsolute-value]
                    abs(preferred_time - pfx->pfx_pltimeexpire) > rai->rai_clockskew) {
                    ^
usr.sbin/rtadvd/rtadvd.c:1291:7: note: remove the call to 'abs' since unsigned values cannot be negative
                    abs(preferred_time - pfx->pfx_pltimeexpire) > rai->rai_clockskew) {
                    ^~~
usr.sbin/rtadvd/rtadvd.c:1324:7: error: taking the absolute value of unsigned type 'unsigned int' has no effect [-Werror,-Wabsolute-value]
                    abs(valid_time - pfx->pfx_vltimeexpire) > rai->rai_clockskew) {
                    ^
usr.sbin/rtadvd/rtadvd.c:1324:7: note: remove the call to 'abs' since unsigned values cannot be negative
                    abs(valid_time - pfx->pfx_vltimeexpire) > rai->rai_clockskew) {
                    ^~~
2 errors generated.

These warnings occur because both preferred_time and
pfx->pfx_pltimeexpire are uint32_t's, so the subtraction expression is
also unsigned, and calling abs() is a no-op.

However, the intention was probably to look at the absolute difference
between the two unsigned quantities. Introduce a small static function
to clarify what we're doing, and call that instead.

[hrs]

Yes, the intention of this code was to obtain absolute value of the difference as you pointed out. I think the patch is good.

[dim]

Closed by commit rS274898 (authored by @dim).